Process for the preparation of unsaturated acids from unsaturated aldehydes

ABSTRACT

Methacrylic acid or acrylic acid are produced by the oxidation of methacrolein or acrolein, respectively, with molecular oxygen in the vapor phase in the presence of a catalytic oxide of molybdenum, phosphorus, arsenic, a rare earth element or mixture thereof, oxygen, and optionally, at least one of Ag, Tl, Rh, Pd, Ru, Pt, Cd, Al, Au, Cu, alkaline earth metal, Cl, and NH 4 .

BACKGROUND OF THE INVENTION

A number of catalysts are known to be effective for the oxidation ofacrolein or methacrolein to acrylic acid or methacrylic acid,respectively. However, the yields obtained using the catalysts for thepreparation of methacrylic acid are low. West German Provisional Pat.No. 2,048,620 discloses catalysts containing the oxides of molybdenum,phosphorus, and arsenic for the oxidation of methacrolein and acroleinto methacrylic acid and acrylic acid, respectively. U.S. Pat. No.3,761,516 discloses catalysts containing oxides of molybdenum, arsenicand phosphorus on a support, especially Al₂ O₃, having externalmacropores and a surface not greater than 2 m² /g.

The present invention is a result of a search for more efficient anddesirable catalysts for the production of acrylic acid and methacrylicacid. Unexpectedly higher yields of and selectivities to acrylic acidand methacrylic acid are obtained by the vapor phase oxidation ofacrolein and methacrolein, respectively, with molecular oxygen in thepresence of the new and useful catalysts of the present invention.

SUMMARY OF THE INVENTION

It has been discovered according to the present invention in the processfor the preparation of acrylic acid or methacrylic acid by the oxidationof acrolein or methacrolein, respectively, with molecular oxygen in thevapor phase at a reaction temperature of about 200° to about 500° C inthe presence of an oxide catalyst, and optionally in the presence ofsteam, the improvement comprising

Using as a catalyst a catalyst of the formula

    X.sub.a Y.sub.b Mo.sub.12 P.sub.c As.sub.d O.sub.x

wherein

X is a rare earth element or a mixture thereof;

Y is at least one of Ag, Tl, Rh, Pd, Ru, Pt, Cd, Al, Au, Cu, alkalineearth metal, Cl, and NH₄ ;

wherein

a is 0.001 to 10;

b is 0 to 10;

c is 0.01 to 5;

d is 0.01 to 5;

x is the number of oxygens required to satisfy the valence states of theother elements present.

The surprisingly advantageous catalysts of this invention give improvedyields of acrylic acid and methacrylic acid from acrolein andmethacrolein, respectively, in an efficient, convenient, and economicalmanner at a relatively low temperature.

The most significant aspect of the present invention is the catalystemployed. The catalyst may be any of the catalysts delineated by theabove formula. The catalysts can be prepared by a number of differenttechniques described in the art, such as coprecipitation of solublesalts and calcination of the resulting product.

The catalysts of the invention have preferred limitations on theircomposition. Preferred are catalysts wherein a is 0.001 to 3, catalystswherein b is 0.001 to 3, and catalysts wherein b is zero.

Especially preferred are catalysts wherein X represents cerium, erbium,or a mixture of rare earth elements consisting essentially of Ce, La, Ndand Pr. Catalysts of particular interest are described wherein Y issilver, thallium or copper.

In the catalyst preparations, the various elements of the catalyst arecombined, and the final product is calcined to obtain the catalyst. Anumber of methods of combining the elements of the catalyst andcalcining the resultant product are known to those of skill in the art.In the broad concept of the invention, the particular method ofpreparing the catalysts is not critical.

There are, however, methods of preparing the catalysts that have beenfound to be preferred. One preferred preparation involves thepreparation of the catalysts in an aqueous slurry or solution ofmolybdenum, arsenic, and/or phosphorus containing components, and addingthe remaining components; evaporation of this aqueous mixture; andcalcination of the resulting catalysts. Suitable molybdenum compoundsthat may be employed in the preparation of the catalysts delineated bythe above formula include molybdenum trioxide, phosphomolybdic acid,molybdic acid, ammonium heptamolybdate and the like. Suitable phosphoruscompounds that may be employed in the preparation of the catalystsinclude ortho-phosphoric acid, metaphosphoric acid, triphosphoric acid,and phosphorus halides or oxyhalides. The remaining components of thecatalysts may be added as oxide, acetate, formate, sulfate, nitrate,carbonate, oxyhalide, or halide and the like.

Excellent results are obtained by refluxing phosphoric acid, an arseniccontaining compound, and molybdenum trioxide, or ammonium heptamolybdatein water for about one-half hour to 3 hours, however, commercialphosphomolybdic acid may be effectively utilized; adding the remainingcomponents to the aqueous slurry and boiling to a thick paste; drying at110° to 120° C in air; and calcining the resulting catalysts.

The calcination of the catalyst usually is accomplished by heating thedry catalytic components at a temperature of about 200° to about 700° C.The preferred procedure of the invention is wherein the catalyst iscalcined at a temperature of 325° to 425° C.

The reactants of the reaction of the invention are methacrolein oracrolein and oxygen. Molecular oxygen is normally supplied to thereaction in the form of air, but oxygen gas could also be employed.About 0.5 to about 4 moles of oxygen are normally added per mole ofmethacrolein.

The reaction temperature may vary as different catalysts are employed.Normally, temperature of about 200° to about 500° C are employed withtemperature of 250° C to 370° C being preferred.

The catalyst may be used alone or a support could be employed. Suitablesupports include silica, alumina, Alundum, silicon carbide, boronphosphate, zirconia and titania. The catalysts are conveniently used ina fixed-bed reactor using tablets, pellets or the like or in a fluid-bedreactor using a catalyst having a particle size of less than about 300microns. When a fluid-bed reactor is employed, preferred catalysts arein the form of microspheroidal particles. The contact time may be as lowas a fraction of a second or as high as 20 seconds or more. The reactionmay be conducted at atmospheric, superatmospheric or subatmosphericpressure, with absolute pressures of about 0.5 to about 4 atmospheresbeing preferred.

Excellent results are obtained using a coated catalyst consistingessentially of an inert support material having a diameter of at least20 microns and an outer surface and a continuous coating of said activecatalyst on said inert support strongly adhering to the outer surface ofsaid support. The special coated catalyst consists of an inner supportmaterial having an outside surface and a coating of the active catalyticmaterial on this outside surface. These catalysts can be prepared by anumber of different methods.

The support material for the catalyst forms the inner core of thecatalyst. This is an essentially inert support and may havesubstantially any particle size although a diameter of greater than 20microns is preferred. Especially preferred in the present invention foruse in a commercial reactor are those supports which are spherical andwhich have a diameter of about 0.2 cm. to about 2 cm. Suitable examplesof essentially inert support materials include: Alundum, silica,alumina, alumina-silica, silicon carbide, titania and zirconia.Especially preferred among these supports are Alundum, silica, aluminaand alumina silica.

The catalysts may contain essentially any proportions of support andcatalytically active material. The limits of this relationship are onlyset by the relative ability of the catalyst and support material toaccommodate each other. Preferred catalysts contain about 10 to about100 percent by weight of catalytically active material based on theweight of the support.

The preparation of these coated catalysts can be accomplished by varioustechniques. The basic method of preparing these catalysts is bypartially wetting the support material with a liquid and then contactingthe support material with a powder of the catalytically active materialand gently agitating the mixture until the catalyst is formed. Thegentle agitation is most conveniently accomplished by placing thepartially wet support in a rotating drum or jar and adding the powderedactive catalytic material.

Using the catalysts of the invention in the preparation of methacrylicacid or acrylic acid, excellent yields are obtained in a convenientreaction with low amounts of byproducts.

SPECIFIC EMBODIMENTS Comparative Examples A to C and Examples 1 to 20:Comparison of catalysts containing promoters of invention with basecatalyst in the preparation of methacrylic acid.

A 20 cc. fixed-bed reactor was constructed of a 1.3 cm. stainless steeltubing. Catalysts prepared as described below were charged to thereactor and heated to the reaction temperature under a flow of air and afeed of methacrolein/air/nitrogen/steam of 1/5.7/4.6/8.7 was fed overthe catalyst at an apparent contact time of 2 to 4 seconds. The reactorwas run under the reaction conditions for 1 to 6 hours and the productwas collected and analyzed.

Comparative Example A and Examples 1 to 9

The catalysts were prepared as follows:

Comparative Example A 25% Mo₁₂ P₁ As₀.5 O_(x) + 75% Alundum

A solution was prepared consisting of 211.88 grams of ammoniumheptamolybdate, (NH₄)₆ Mo₇ O₂₄.4H₂ O, (1.2 mole Mo), 500 mls. distilledwater at 60° C and 7.94 grams of ammonium arsenate NH₄ H₂ AsO₄, (0.05mole As) as solution in 25 mls. distilled water. A white precipitateformed which was heated to about 100° C for 2 hours. To this mixture wasadded 11.53 grams of 85% solution phosphoric acid (0.10 mole P).One-half hour later 5.0 grams of hydrazine hydrate was added. The slurrywas evaporated to a thick paste, dried overnight in an oven at 110° to120° C, and ground and screened to less than 80 mesh. This powder wascoated on Norton 1/8 inch SA 5223 Alundum balls by taking 50 grams ofAlundum, partially wetting the Alundum with 1.8 grams of water andadding 16.7 grams of active catalyst prepared above in five equalportions. During and after each addition, the Alundum was rolled in aglass jar. The powder was evenly coated onto the surface of the Alundumand the final product was dried. A hard uniform material was obtainedthat consisted of an inner core of the Alundum support with thecontinuous, strongly adhering coat of the powder on the outside surfaceof the support. The material was then calcined for 1 hour at 370° C in40 ml/min. air to form the active catalyst.

EXAMPLE 1 25% (Rare earth mixture)₀.25 Mo₁₂ P₁ As₀.5 O_(x) + 75% Alundum

A solution was prepared consisting of 105.9 grams of ammoniumheptamolybdate, (NH₄)₆ Mo₇ O₂₄.4H₂ O, (0.6 mole Mo), 700 mls. ofdistilled water at 60° C and 4.0 grams of ammonium arsenate NH₄ H₂ AsO₄,(0.025 mole As) as solution in 25 mls. of water. A white precipitateformed which was heated at 100° C about one-half hour. To this mixturewas added 4.4 grams of Moly Corp. rare earth chloride mixture (ProductCode No. 4700) consisting of 48% CeO₂, 33% La₂ O₃, 13% Nd₂ O₃, 4.5% Pr₆O₁ and 1.5% other rare earth elements calculated as oxides. To thissolution was added 5.8 grams of 85% solution phosphoric acid, H₃ PO₄(0.05 mole P). One-half hour later 2.5 grams of hydrazine hydrate wereadded. The slurry was evaporated to a thick paste, dried overnight in anoven at 110° to 120° C, and ground and screened to less than 80 meshsize. The catalyst was then coated to a 25% active level on 1/8 inch SA5223 Alundum balls. Calcination was the same as in Comparative ExampleA.

EXAMPLES 2 to 7 Preparation of the Catalysts 25% X_(a) Y_(b) Mo₁₂ P₁As₀.5 O_(x) + 75% Alundum

Various catalysts of the invention were prepared. The catalysts wereprepared according to the procedure of Example 1, using 105.9 grams ofammonium molybdate, 700 mls. of 60° C. distilled water and 4.0 grams ofammonium arsenate in solution of 25 mls. of water. The catalyticcomponents delineated by X and/or Y were added immediately preceding theaddition of 5.8 grams of 85% phosphoric acid and 2.5 grams of hydrazinehydrate. To prepare the catalysts, the following compounds and amountswere used:

    ______________________________________                                        Example                                                                              Element    Compound        Amount, g.                                  ______________________________________                                        2      Ce.sub.0.25                                                                              cerium acetate  4.31                                        3      Ce.sub.0.1 cerium acetate  1.72                                        4      Er.sub.0.25                                                                              erbium acetate  5.21                                        5      Rare       Rare earth chloride                                                                           1.77                                               earth      mixture (Moly Corp.                                                mixture.sub.0.1                                                                          No. 4700)                                                          Cu.sub.0.25                                                                              copper acetate  2.48                                        6      Rare       Rare earth chloride                                                                           4.43                                               earth      mixture                                                            mixture.sub.0.25                                                              Tl.sub.0.05                                                                              thallium acetate                                                                              0.66                                        7      Ce.sub.0.15                                                                              cerium acetate  2.59                                               Ag.sub.0.1 silver acetate  0.85                                        ______________________________________                                    

EXAMPLE 8 25% Rare earth mixture₀.25 Cu₀.05 Mo₁₂ P₁ As₀.5 0_(x) + 75%Alundum

This catalyst was prepared in the same manner described in Example 1,except 34.25 grams of ammonium heptamolybdate, 1.28 grams of ammoniumarsenate, 1.43 grams of rare earth chloride mixture, 0.161 grams ofcopper acetate, 1.88 grams of 85% phosphoric acid and 0.8 grams ofhydrazine hydrate were employed.

EXAMPLE 9 25% Rare earth mixture₀.25 Ag₀.1 Mo₁₂ P₁ As_(O).5 O_(x) + 75%Alundum

This catalyst was prepared in the same manner described in Example 8,except that 0.269 grams of silver acetate were employed.

Comparative Examples B, C, and Examples 10 to 20

The results of the experiments in the oxidation of methacrolein toproduce methacrylic acid are shown in the TABLE below. The followingdefinitions are used in measuring the carbon atoms in the feed and theproducts. ##EQU1##

                                      TABLE                                       __________________________________________________________________________    Performance of Catalysts Containing Promoters of Invention Compared With      Base Catalyst in the Preparation of Methacrylic Acid                          __________________________________________________________________________                         Reaction                                                                            Results, %                                         Example                                                                             Catalyst       Temp.° C                                                                     Methacrylic Acid                                                                        Acetic Acid                                                                           Total Conversion                                                                        Selectivity            __________________________________________________________________________    Comp. B                                                                             25%Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x +                                                    326   19.6      0.5     26.0      75.0                         75%            Alundum                                                  C     25%Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x +                                                    350   43.0      1.9     56.0      76.0                         75% Alundum                                                             10    25%R.E..sub.0.25 Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x                                        327   55.0      4.5     77.0      72.0                         + 75% Alundum                                                           11    25%Ce.sub.0.25 Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x +                                        338   54.3      5.0     76.0      72.0                         75% Alundum                                                             12    25%Ce.sub.0.1 Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x +                                         350   54.0      5.1     78.0      70.0                         75% Alundum                                                             13    25%Er.sub.0.25 Mo.sub.12 P.sub.1 As.sub.0.5 O.sub.x +                                        322   60.0      5.6     83.4      71.8                         75% Alundum                                                             14    25%R.E..sub.0.1 Cu.sub.0.25 Mo.sub.12 P.sub.1                                                321   71.4      3.7     85.7      83.3                         As.sub.0.5 O.sub.x +75% Alundum                                         15    25%R.E..sub.0.1 Cu.sub.0.25 Mo.sub.12 P.sub.1                                                329   73.7      4.8     91.5      80.6                         As.sub.0.5 O.sub.x +75% Alundum                                         16    25%R.E..sub.0.1 Cu.sub.0.25 Mo.sub.12 P.sub.1                                                343   70.4      6.3     93.6      75.3                         As.sub.0.5 O.sub.x +75% Alundum                                         17    25%R.E..sub.0.25 Tl.sub.0.05 Mo.sub.12                                                       353   55.4      4.4     76.1      72.7                         P.sub.1 As.sub.0.5 O.sub.x +75% Alundum                                 18    25%Ce.sub.0.15 Ag.sub.0.1 Mo.sub.12 P.sub.1                                                  358   56.4      6.8     85.7      66                           As.sub.0.5 O.sub.x +75% Alundum                                         19    25%R.E..sub.0.25 Cu.sub.0.05 Mo.sub.12 P.sub.1                                               330   56.8      4.7     79.3      71.6                         As.sub.0.5 O.sub.x +75% Alundum                                         20    25%R.E..sub.0.25 Ag.sub.0.1 Mo.sub.12 P.sub.1                                                307   54.1      5.1     74.6      72.5                         As.sub.0.5 O.sub.x +75% Alundum                                         __________________________________________________________________________

We claim:
 1. The catalyst composition described by the formula

    X.sub.a Y.sub.b Mo.sub.12 P.sub.c As.sub.d O.sub.x

wherein X is a rare earth element or a mixture thereof; Y is at leastone of Ag, Tl, Rh, Pd, Ru, Pt, Cd, Al, Au, Cu, alkaline earth metal, Cl,and NH₄ ;wherein a is 0.001 to 10; b is 0 to 10; c is 0.01 to 5; d is0.01 to 5; x is the number of oxygens required to satisfy the valencestates of the other elements present.
 2. The catalyst of claim 1 whereinX is cerium, erbium, or a mixture of rare earth elements consistingessentially of Ce, La, Nd, and Pr.
 3. The catalyst of claim 1 wherein Xis a mixture of rare earth elements consisting essentially of Ce, La, Ndand Pr.
 4. The catalyst of claim 1 wherein X is cerium.
 5. The catalystof claim 1 wherein X is erbium.
 6. The catalyst of claim 1 wherein Y isat least one of silver, thallium and copper.
 7. The catalyst of claim 1wherein Y is silver.
 8. The catalyst of claim 1 wherein Y is thallium.9. The catalyst of claim 1 wherein Y is copper.
 10. The catalyst ofclaim 1 wherein b is zero.
 11. The catalyst of claim 1 wherein a is0.001 to
 3. 12. The catalyst of claim 1 wherein b is 0.001 to
 3. 13. Thecatalyst of claim 1 which is coated on an inert support.
 14. Thecatalyst of claim 13 consisting essentially of an inert support materialhaving a diameter of at least 20 microns and an outer surface and acontinuous coating of said active catalyst strongly adhering to theouter surface of said support.
 15. The catalyst of claim 14 wherein theactive catalyst is about 10 to about 100 percent by weight of the inertsupport.
 16. The catalyst of claim 14 wherein the support is selectedfrom the group consisting of silica, alumina, alumina-silica, siliconcarbide, titania and zirconia.
 17. The catalyst of claim 14 wherein theparticle size of the inert support is 0.2 cm. to 2 cm.